Direct-Reading
Gas Detection Instruments
Presented by
Michael D. Shaw
“DIRECT-READING”
Means that instrument provides display
in engineering units (usually ppm or percent)
Implication (not always true) that unit is reading in real time
GASES OF INTEREST
Combustible
Oxygen (deficiency and enrichment)
Toxic
MOST COMMON TECHNOLOGIES EMPLOYED
IN INDUSTRIAL HYGIENE APPLICATIONS
CATALYTIC SENSORS
(also known as Pellistors)
Theory - Target gas is oxidized on catalytic element
The change in
temperature causes
a change in
resistance that is
measured by the
meter
Application – combustible gases
CATALYTIC SENSORS
Advantage
Long life
Disadvantages
Different responses for each combustible gas
Can be poisoned
Most require at least 10% oxygen to work properly
Limited to percent-level detection
METAL OXIDE SEMICONDUCTOR
(Also known as solid state, Figaro, or Taguchi sensors)
Theory – Target gas reacts with MOS (SnO2 ) and
changes its resistance – as measured by the meter
Application - Nearly all oxidizable gases
METAL OXIDE SEMICONDUCTOR
Advantage
Inexpensive
Disadvantages
Not selective, and this is often misrepresented
Affected by humidity
Not truly analytical—better as “go/no-go” detectors
NON-DISPERSIVE INFRARED (NDIR)
Theory – Target gas absorbs infrared light at a particular wavelength
Via Beer’s Law
calculations —
concentration
can be determined
Application – Hundreds of compounds, especially organics
BEER’S LAW ( FOR EXTRA CREDIT )
"The deeper the glass,
The darker the brew,
The less of the incident
Light that gets through"
I = I0ekP
I = intensity of light striking the detector
I0 = measured signal at zero concentration of target gas
k = system dependent constant
P = concentration of target gas
e = base for natural logarithms
NON-DISPERSIVE INFRARED (NDIR)
Advantage
Extremely versatile
Disadvantages
Expensive
Can be fragile and complicated to service
PHOTOIONIZATION DETECTOR (PID)
Theory - Ultraviolet light ionizes the target gas
Current produced is proportional to concentration
Application – Volatile Organic Compounds (VOC’s)
PHOTOIONIZATION DETECTOR (PID)
Advantage
Detects a wide range of volatile organic compounds
Quite effective when used in conjunction
with chromatographic column → Portable GC’s
Disadvantages
Nonselective among organic vapors below
ionization potential of lamp
Affected by high humidity
UV lamps are expensive
ELECTROCHEMICAL SENSORS
Theory – Target gas is adsorbed on an electrocatalytic sensing
electrode, after passing through a diffusion medium, and is
electrochemically reacted.
The current produced
by this reaction is
directly proportional
to the gas
concentration.
Applications
Br2, CO, Cl2,
ClO2, C2H4,
ethylene oxide
HCHO, H2, hydrazine, HBr, HCl, HCN, H2S, NO, NO2, O3,
propylene oxide, SO2, and oxygen-- among others
ELECTROCHEMICAL SENSORS
Advantages
Inexpensive
Linear output
Can be miniaturized
Disadvantages
Prone to interferences in unskilled hands
Affected by temperature— readily correctable
WHAT IS THE MOST IMPORTANT THING
IN GAS DETECTION WHEN USING
DIRECT-READING INSTRUMENTS?
PROPER CALIBRATION!!
Without a clean zero gas and an accurate verified
calibration standard—there is no point in doing any gas
detection
MONITORING PRODUCTS
OFFERED BY
PORTABLE ANALYZER
CONTINUOUS MONITORING SYSTEM
SINGLE POINT APPLICATION
CONTINUOUS MONITORING SYSTEM
TWO POINT APPLICATION
CONTINUOUS MONITORING SYSTEM
MULTI-POINT (THREE OR MORE) APPLICATION
ARC-MAX®
DATA ACQUISITION, ARCHIVING,
AND REPORTING
ARC-MAX® MAIN SCREEN
ARC-MAX® SHIFT REPORT
ARC-MAX® TRENDING
ARC-MAX® ALARM LOG
For Survey Applications,
We Recommend Our Nomad Data Logger
12-bit resolution
Models available for
0-100mV, 0-2.5V
and 4-20mA inputs
Self-contained units
also available for
temperature, humidity
and pH
Sampling rate adjustable—1 second to 10 hours in 1 second intervals
Storage capacity—16336 readings
Excellent software included
Visualize your measurement session with an auto-scaling time history graph
Time history graph detail screen
Zoomed in on particular portion of the curve
Ability To View All Individual Data Points
CONCLUDING REMARKS
We've looked at the most widely used operating
principles in direct-reading gas detection instruments
We've touched upon the importance of calibration
We've examined two approaches to data acquisition
SPECIAL THANKS TO
PROFESSOR GEORGE BYRNS